1. Field of the Invention
The present invention relates to a high zoom-ratio zoom lens system that is suitable as an imaging optical system, mainly for use with an image sensor, which includes a wide-angle range of an angle-of-view of 70 degrees or greater.
2. Description of Related Art
In recent years, in the field of compact digital cameras, the demand for high zoom-ratio zoom lens systems that include a wide-angle range has increased. On the other hand, miniaturization of the pixel pitch has increased, so that a high resolution is demanded in the optical system. It is common to employ, in imaging lenses for use in mainly compact digital cameras, a retractable-barrel mechanism that uses a so-called multi-stage barrel that reduces the air-distance between lens groups at a fully-retracted (accommodated) state in order to achieve a slimmer (i.e., shorter in the optical axis direction) camera body. In addition to reducing the thickness of each lens group in a lens system that is suitable for such a retractable-barrel mechanism, it is also necessary for the frontmost lens to be small in diameter, and furthermore, for this lens system to be designed with consideration of the mechanical restrictions in regard to the overall length thereof.
As an example of a zoom lens system for use in a high zoom-ratio compact digital camera, a zoom lens system such as that disclosed in Japanese Unexamined Patent Publication No. 2006-308957 is known in the art, in which a relatively small number of lens elements are utilized, further miniaturization is achieved, and a wide angle-of-view of approximately 78 degrees is achieved at the short focal length extremity (wide-angle extremity). However, the zoom ratio is approximately 4:1 through 7:1, which is unsatisfactory.
As an example of a zoom lens system having a zoom ratio of approximately 10:1, zoom lens system such as that disclosed in Japanese Patent No. 3,304,518 is known in the art. However, the overall length of the zoom lens system with respect to the focal length and the zoom ratio is long, hence, this zoom lens system is not sufficiently miniaturized. Furthermore, in a zoom lens system disclosed in Japanese Unexamined Patent Publication No. 2007-122019, a wide angle-of-view is not achieved and the zoom ratio is insufficient, and the overall length of the zoom lens system is also long, and hence, this zoom lens system is also not sufficiently miniaturized.